The invention relates to an optoelectronic coupling device for transmitting digital signals from an input to an output, which input and output are electronically isolated from each other.
To transmit digital signals like telegraph signals transmission systems have been used which are composed of units having different signal characteristics. Because of these different signal characteristics, it is often necessary to isolate parts of the transmission system from each other electrically. It is well known to employ different kinds of relays for this purpose; however, relays have been replaced more and more by optoelectronic coupling devices which are less sluggish and which have less current consumption in addition to other advantages over relays.
One example of this kind of optoelectronic coupling device is known from the U.S. Pat. No. 3,810,034 to Brunsch. This U.S. patent discloses an apparatus for the transmission of direct current signals to an output circuit from an input circuit electrically isolated therefrom by means of optoelectronic coupling elements. To improve the transmission performance for high signaling speeds and to reduce distortions, an operational amplifier is connected to the output of the optoelectronic coupling circuit. Furthermore, both inputs of the operational amplifier are connected via a potentiometer, the tap of which having the same ground connection as the operational amplifier. By means of this potentiometer, the d.c. signals to be transmitted can be balanced symmetrically.
Such an adjustable potentiometer has to be readjusted manually. In addition to this disadvantage, the known optoelectronic signal coupler may not be well suited if optoelectronic circuits with phototransistors instead of photodiodes will be used; since phototransistors have, to a high degree, differing switching times, if they are switched on or cut off. The different characteristics, in switch-on time and cut-off time depend on a cut-off delay of a saturated phototransistor which is longer than the delay-time during the switch-on operation.
It is, therefore, an object of this invention to provide an improved optoelectronic coupling device for transmitting digital signals, the coupling stage of which device is built up with phototransistors.
Another object of the invention is to provide an optoelectronic coupling device which is to be operated at high transmission speeds.
A further object of the invention is to provide an optoelectronic coupling device which is automatically readjusted and which is operable with different signal levels, especially in that combination of different signal levels at the input and the output.